High molecular weight growth factor (HMW GF) was efficiently absorbed by trimethylsily-controlled pore glass beads (TMS-CpG beads) from the bulk urine of a patient bearing a glioblastoma multiforme. After a second cycle of TMS-CpG absorption, HMW GF had a molecular weight of approximately 28,000 using Bio-Gel P-100 chromatography. Following an estimated 95% surgical resection of tumor, no appreciable growth factor activity of this sort was detectable in a comparable 48-hour urine collection. This brain tumor-associated HMW GF activity contained only about 20% of the radioreceptor activity of the standard small molecular weight human epidermal growth factor (SMW hEGF) while maintaining full immunoreactivity to the SMW hEGF. Consequently, the HMW GF associated with brain tumor burden in this patient appears similar to the high molecular weight form of human EGF (HMW hEGF) previously reported in low concentration in normal human urine. Thus, the HMW GF may be of host cell, rather than uniquely of tumor cell, origin. The process of horizontal recruitment in transplantation tumor biology is defined as the induction of malignancy in adjacent, presumably normal, host cells of tumor-bearing animals. We have reported a human chondroblastic tumor in a nu/nu mouse expressing a mixed histologic composition. The biparate tumor was injected into nu/nu rats. A growth factor of high molecular weight was demonstrated in the urine of the tumor-bearing rats which may be of rat or human origin.